Dry eye disease, or Keratoconjunctivitis Sicca (KCS) is one of the most frequently established diagnoses in ophthalmology. Current estimates hold that roughly 40-60 million people in the United States exhibit dry eye symptoms. The lack of accurate statistical data about the occurrence of dry eye is due largely to a lack of state-of-the-art diagnostic equipment. A more disturbing trend, however, is the misdiagnosis of dry eye or its escape from early detection altogether, since symptomatic patients are not always easily identified.
Pursuing more effective diagnosis will strengthen the paradigm of ophthalmic care, a fact recognized by the pharmaceutical industry. The first prescription pharmaceuticals for treating dry eye are now appearing on the market, with more on the way, and yet the methods for diagnosis and monitoring treatment remain problematic.
There is no “gold standard” test that both diagnoses dry eye and monitors the effectiveness of treatment efforts. One popular method is a matrix of subjective observation of symptoms and objective tests (such as Schirmer testing, staining techniques and tear break-up time), none of which is specific to the detection of dry eye or the measurement of its severity. Considering recent pharmaceutical advancements aimed at treating dry eye, timely and parallel advancements in diagnostic technologies are needed.
The osmolarity of a tear—the degree of dissolved solids therein—is popularly accepted by experts in the field as an indicator of the presence and severity of dry eye. The instrument most commonly associated with the measurement of tear osmolarity is the osmometer; however, technical limitations have restricted the use of tear osmometers to primarily research environments.
An osmometer is a device that measures the concentration of dissolved solutes in a liquid, such as water. Though it is widely used in other fields, osmometers are used in medicine in applications such as determining osmol gap in toxicology and trauma cases, monitoring mannitol treatment infusions, and monitoring the absorption in glycine ingestion with irrigation fluids in surgical procedures, among others.
Despite the suitability of this technology for measuring tear osmolarity, current devices present certain limitations that prevent their widespread use in a clinical environment. The most prevalent problem has to do with sample size.
Nearly all commercially available osmometers are designed (and perhaps technologically limited) to measure milliliter-size samples. Tear samples extracted from patients tend to be in the nanoliter volumes, and further complicating matters, dry eye patients generally have fewer tears, making handling of samples even more difficult. Osmometers designed to measure nanoliter sample sizes are not available commercially and are too cumbersome for practical use in a clinical environment. The result is that practicing ophthalmologists are left with a haphazard methodology and inadequate tools to accurately detect this prevalent condition.
Dry eye disease is a complex group of diseases characterized by a decreased production of one or more of the three components of the tear film: the lipid layer, the aqueous layer, and the mucin layer. A deficiency in one of the tear film components may lead to a loss of the tear film stability. Normal sight relies on a moist ocular surface and requires a sufficient quality of tears, normal composition of the tear film, regular blinking and normal lid closure as prerequisites. If left untreated, dry eye syndrome can cause progressive pathological changes in the conjunctival and corneal epithelium, discomfort, corneal ulceration's and even ultimately lead to blindness.
Standard treatment has been tear replacement therapy, which attempts to either mimic the human tear film or present a more sophisticated hypo-osmolar version of the tear film. Unfortunately, as dry eye syndrome progresses beyond the mild stage, this common therapy becomes less effective. Further, these treatments do not address the etiology of dry eye.
The precise mechanisms that give rise to dry eye are currently unknown and have been the subject of debate over the years. Recently, several different mechanisms have been proposed as a possible etiology of dry eye, with a general ideology that it is usually caused by a problem with the quality of the tear film that lubricates the ocular surface. More recent research has proposed that dry eye may be a result of a decrease in hormonal status with aging (being more prominent in postmenopausal women), or have an immune basis and acquired inflammatory condition of the ocular surface. Other causes of dry eye symptoms can occur from certain medications (e.g., antihistamines, beta-blockers), associations with certain systemic inflammatory diseases (e.g., rheumatoid arthritis), mechanical causes (e.g., incomplete closure of eyelids), infectious causes (e.g., viral infections) and certain neurological causes (e.g., LASIK procedures). Despite recent gains in knowledge of possible pathogenic factors of dry eye, there has been a lack of consensus as to the appropriate diagnostic criteria, the specific aims of objective diagnostic testing, the role subjective symptoms play in diagnosis, and the interpretation of results.
The symptoms of dry eye vary considerably from one individual to another. Most patients complain of a foreign body sensation, burning and general ocular discomfort. The discomfort is typically described as a scratchy, dry, sore, gritty, smarting or burning feeling. Discomfort is the hallmark of dry eye because the cornea is richly supplied with sensory nerve fibers.
Despite its high prevalence, dry eye is not always easy to diagnose. The vast majority of patients have symptoms that are mild to moderate in severity. Although these patients are genuinely suffering discomfort, objective signs of dry eye may be missed, and without proper diagnosis, patients may not receive the attention and treatment that this condition warrants. The signs and symptoms of dry eye can be misinterpreted as evidence of other conditions, such as infectious, allergic, or irritative conjunctivitis. Given these complications in diagnosis, it is estimated that the diagnosis rate of dry eye is approximately 20%.
Diagnosis of dry eye typically begins with clinical examination. A Schrimer test is usually performed where standardized strips of filter paper are placed at the junction between the middle and lateral third of the lower lid. If less than 5 millimeters has been wetted after 5 minutes, there is reason to believe aqueous tear deficiency is present. Though the test is quick, inexpensive and results are available immediately, it provides only a rough estimate and is unreliable in moderate dry eye.
Dye staining is another method of diagnosing dry eye, with either fluorescein or Rose Bengal, and a trained physician can look for patterns under slit lamp observation indicating dryness. Another test, tear break-up time, is a measure of the stability of the tear film. A normal tear film begins to break up after approximately 10 seconds, and this time is reduced in patients with dry eye.
The osmometer generally used in measuring tear osmolarity is the Clifton Direct Reading Nanoliter Osmometer (Clifton Technical Physics, Hartford, N.Y.) developed in the 1960's. Although not necessarily originally intended for use in measuring tears, it is one of the few instruments capable of measuring nanoliter volumes of solution and has found its way into ophthalmology.
The Clifton Osmometer was produced in limited quantities over the years, and is not routinely used outside a research laboratory. It is based on the well-known measurement technique called freezing point depression. The Clifton Osmometer measures the osmolarity of a sample by measuring the freezing point depression. In freezing point depression measurements, water (which normally freezes at 0° C.), experiences a depression in its freezing temperature in presence of dissolved solutes, the mathematical relationship of which is defined by Raoult's Law.
Though the test can be accurate, it requires a very skilled operator to make the measurement. The test monitors the depression in freezing temperature by examining a fractional volume of a teardrop under a microscope. Due to its limitations and lack of availability, there appears to be only a few units left in the field. Furthermore each measurement can take over fifteen minutes, which, coupled with the small sample volumes, make the use of the Clifton Osmometer an extremely tedious and inconvenient process. The amount of time required and the operating skill demanded are unacceptable to a busy practice or clinic, even if the units were available.
There is a need for simple and accurate sensors and systems that can diagnose and monitor treatment efforts for dry eye disease.